The present invention concerns an electrochemical sensor in the shape of a tongue intended to measure, using a reactant which is deposited beforehand on its working electrode, the concentration of a constituent in a sample solution or liquid or natural or biological origin in a reproducible manner from one sensor to another sensor.
The invention concerns more particularly sensors of this type which are used for the medical follow-up of a patient requiring frequent measuring of a constituent in a biological body fluid so as to adapt a treatment as a function of a reference value, such as the blood glucose level for a person suffering from diabetes.
During the last ten years, the generally disposable electrochemical sensors, intended for biological measurements, have experienced considerable development aimed at improving qualities, such as sensitivity, reliability, speed of response or ease of use by a user. Generally speaking, these sensors are formed by an insulating support of small dimensions, supporting at least two conducting strips electrically separated and able to be connected at one end to an electronic measuring device, said conducting strips being covered with a film into which two windows are cut exposing portions of strip respectively forming the reference electrode and the working electrode on which there is immobilised a determined quantity of specific reactant of the constituent whose concentration one wishes to determine. After having deposited the sample to be analysed on the measuring zone, for example a drop of blood, the measurement is effected indirectly by exploiting an electric signal generated by the interaction between said specific reactant and said constituent.
This exploitation of the electric signal consists generally in conductometric, voltainmetric, amperometric, coulometric or polarographic measurements allowing an electronic measuring device to interpret said signal and to display the concentration of the constituent in a determined mode (mg/dl, mmol/l) directly on a screen. In order for the displayed value to always be the same for a determined concentration, the electric signal supplied by the sensor must not vary as a function of the sensor used, i.e. these sensors must be able to be manufactured at a low cost in series to be disposable via a method which nonetheless guarantees a high degree of reproducibility. The parameters which can be acted on are in particular the dosage precision of the various compounds involved in the composition of the specific reactant, the precision of the quantity of reactant deposited on the working electrode, and the precision of the useful surface of the electrodes, in particular the surface of the working electrode which is actually covered by the specific reactant. It is relatively easy to obtain a high level of precision for the composition of the reactant and for the quantity deposited on the working electrode. Following the teaching of European Patent No. 0 787 984, it is also possible to have a high level of precision as regards the useful surface of the electrodes by having conducting strips which pass through the windows without leaving any portion of the substrate apparent. However, the Applicant has noticed that it is relatively difficult to have a perfect reproducible covering of the useful surface of the working electrode by the specific reactant when it is applied by pipetting in windows of the current shapes, namely rectangular windows or windows in the shape of a half-moon, i.e. in both cases windows having contours with sharp angles.
The material into which the windows are cut is generally a hydrophobic material, such as polyethyleneterephtalate (PET). This hydrophobic phenomenon is in competition with the capillary properties of the specific reactant, so that when the desired quantity of specific reactant is deposited at the centre of the window by pipetting, it is not spread uniformly over the entire useful surface of the working electrode. In particular, very irregular covering is noted at the sharp angles.
In order to overcome this, it is possible to effect the deposition of the specific reactant via pipetting by starting by following the contour of the window as closely as possible the contour and/or by giving the pipette a certain inclination. It can easily be seen that such a method is not applicable to series manufacturing in which the sensors are manufactured in batches on substrates in a plate or strip shape prior to being cut to be packaged individually.
In order to make the contour of the window hydrophilic, pre-treatment with an alcohol has also been tried. Indeed, better wettability of the window is obtained, to the point that the specific reactant also spreads over the PET covering sheet. It is difficult, even impossible, to perform this hydrophilic treatment only in the vicinity of the vertical edges of the covering sheet.
The object of the invention is thus to overcome the drawbacks of this prior art by providing an electrochemical sensor wherein the useful surface of the working electrode is covered in a uniform and reproducible manner by a specific reactant of a constituent present in a solution or a liquid of natural or biological origin, as a result of a shape which is oblong in the direction of the tongue and given at least to the window delimiting the useful surface of the working electrode including said specific reactant.